1. Field of Invention
This invention relates to wireless devices, and in particular to wireless devices utilizing the IEEE 802.16 standard.
2. Related Art
With the explosive growth of wireless technologies there is a growing desire to deploy wireless connectivity over larger areas, such as cities and rural areas, as an alternative to using copper and fiber-based solutions. Wireless technologies are typically less expensive to install and support in most cases, especially in locations where the cost is prohibitive to install physical media or there are right-of-way issues.
The problem, however, is that there have not been effective, standards-based solutions for implementing wireless networks within metropolitan-sized areas. Traditionally, companies install proprietary or Institute of Electrical and Electronics Engineers (“IEEE”) 802.11 (generally known as “802.11”) equipment for wireless connectivity over areas outside the confines of a building. Proprietary systems may typically meet performance and security requirements; however, they tend to be more expensive and risky in terms of long-term support. They also lack interoperability, something that many end users typically demand.
The use of IEEE 802.11-based hardware for metropolitan-sized networks usually decreases costs, but the IEEE 802.11 standard has performance limitations when supporting large numbers of users who need guaranteed bandwidth. In addition, RF interferences are often a significant problem in IEEE 802.11 networks when covering large areas due to license free operation of other devices within the same frequency bands. Often a competitor may install an IEEE 802.11 based network that interferes with other networks, and users suffer due to sporadic, poor performance.
In response to these and other problems, the IEEE 802 group initiated the IEEE 802.16 Working Group to create standards for broadband wireless access in order to offer a high speed/capacity, low cost, and a scalable solution to extend fiber optic backbones. The first IEEE Standard 802.16®, published in April 2002, defines the WirelessMAN™ Air Interface for wireless metropolitan area networks (“MANs”). These systems were meant to provide network access to homes, small businesses, and commercial buildings as an alternative to traditional wired connections.
The IEEE Standard 802.16® (generally known as “802.16”) will support point-to-multipoint architecture in the 10-66 GHz range, transmitting at data rates up to 120 Mbps with transmissions requiring line-of-sight (also generally known as “line-of-site”). Typically an IEEE 802.16 base station connects to a wired backbone and can transmit wirelessly up to 30 miles to a large number of stationary subscriber stations, possibly hundreds.
To accommodate non-line-of-sight access over lower frequencies, the IEEE published IEEE 802.16a in January 2003. The IEEE 802.16a standard operates in the licensed and unlicensed frequencies between 2 GHz and 11 GHz, and defines three optional Physical Layers (“PHYs”), two using orthogonal frequency division multiplexing (“OFDM”), and one using a single carrier. Additionally, the IEEE 802.16 media access control (“MAC”) layer supports many different physical layer specifications, both licensed and unlicensed. Through the IEEE 802.16 MAC layer, every base station may be able to dynamically distribute uplink and downlink bandwidth to subscriber stations using time-division multiple access (“TDMA”). This is a difference from the IEEE 802.11 MAC, with current implementations operating through the use of carrier sensing mechanisms that do not provide effective bandwidth control over the radio link.
In the spring of 2003, one need identified by the IEEE 802.16revD working group was the need to increase the range of the uplink (the communication channel over which subscriber units transmit to a base station), especially for mobile operations. In response to this, communication engineers working on the IEEE 802.16revD WirelessMAN-SCa physical layer set a goal of adding functionality to increase both the range and data-carrying capacity of the baseline single-carrier (time-domain modulated) broadband wireless communication system. Severe constraints were placed on the total output power of subscriber units due to the cost of semiconductor power amplifiers and batteries; therefore, throwing additional power at the problem was not an acceptable solution.
A conventional systems-oriented approach to this problem is to divide the usable uplink bandwidth into narrower bandwidth channels, and allocate a single channel to each user. By funneling the output power of each transmitter into a narrower bandwidth channel (i.e., a narrower communication pipe), the energy per bit (i.e., pressure within a communication pipe) is increased. This, in turn, increases the range (of effective communication flow) on the channel, because it can compensate for extra propagation losses. Moreover, multiple users can be supported without interference by assigning each user to its own narrowband channel. Reducing interference then increases capacity because it enables higher order modulations, such as 64-QAM, to be used when adequate signal-to-noise ratio (“SNR”) is available.
Unfortunately, characteristics (and usage) of terrestrial wireless channels often either prohibit or limit the effectiveness of simply narrowing channel bandwidth. For example, in many unlicensed bands, one must re-use (time share) the same channel for the uplink (subscriber unit to base station channel) and downlink (base station to subscriber unit channel). Moreover, even if the bandwidths could be different on the uplink and downlink, a narrowband wireless channel is much more prone to signal-to-noise fluctuations called fading. Such fades are a consequence of multi-path propagation in a wireless environment. For example, if a version of a sinusoid (at a particular frequency) arrives 180 degrees out of phase with another delayed, reflected version of itself, the signals then cancel, and the SNR is minus-infinity. Broader-band signals tend to suffer less from fading because they are not simple sinusoids; they have enough frequency diversity such that only certain frequency ranges of their signaling spectrum are notched out in the worst case. The fading margin between narrowband and broadband channels can be in the tens of decibels so broadband signaling is typically preferred. Previous attempted solutions applied to single carrier modulation include Code Division Multiple Access (“CDMA”) and frequency hopping.
Some 2nd generation and all 3rd generation cellular telephony standards use CDMA. Cellular CDMA utilizes a technique called direct sequence spread spectrum (“DSSS”) to take a narrowband signal, multiply it by a broadband (but known) ‘chipping’ sequence, and transmit the resulting broadband signal over a channel. Since DSSS uses a great deal of bandwidth to transmit a narrowband signal (and is therefore is spectrally very inefficient), cellular CDMA allows multiple transmitters to simultaneously access the channel by assigning each its own channelizing code, typically called a Walsh code. As a result, CDMA therein has several disadvantages:
1) Even with CDMA's spreading and Walsh code orthogonalization sequences, maintaining orthogonality over all delays in a multipath environment is impossible. As a result, multipath causes individual users to interfere with themselves, creating a ‘self-noise floor’.
2) Due to delay spread and time-synchronizing issues, simultaneous users interfere with each other, creating multiple access interference (“MAI”). Unfortunately, MAI is generated even if users are assigned separate orthogonal Walsh codes.
3) To reduce the impact of MAI, tight power control is necessary in a CDMA system, so that no one transmitter produces interference that would overwhelm the reception of transmissions from other users. Unfortunately, power control messages can consume a considerable amount of system capacity.
4) Multipath-related self-interference and MAI generally restricts signaling to only lower order modulations (e.g., bipolar phase shift keying (“BPSK”) and quadrature phase shift keying (“QPSK”)). When higher throughputs are required, 3G cellular systems using CDMA typically dedicate the entire user bandwidth to a single user. As a result, this severely reduces system capacity.
5) The Rake receiver typically used in CDMA implementations are too complicated for true broadband channels (of several MHz to several ten's of MHz), where a Rake receiver would have to resolve many multipath signals, each requiring a separate ‘Rake finger’.
Other transmission schemes such as Bluetooth® (IEEE 802.15) and the European cellular standard GSM are examples of systems using frequency hopping (“FH”). A FH transmitter funnels its energy into a narrowband channel, but continually hops from one narrowband channel to another, so that it never resides for long on a channel that may be faded. The problem with this scheme is if the receiver hops onto a low SNR channel, its demodulation loops fail, and the data received at that frequency is unusable. To overcome outages due to lost data at bad hop frequencies, low forward error correction (“FEC”) code rates are necessary, and throughputs are subsequently low. Tight time and frequency accuracy are also required if multiple users are to operate simultaneously (with different hopping patterns) and not collide on one of the frequencies at the receiver.
Another problem with this scheme is the excessive signaling overhead required by FH. This is due to all of the guard bands and training sequences that FH requires at each hop, to ‘learn’ the channel and avoid other users. Additionally, demands on dynamic range and the speed of a receiver's carrier tracking and automatic gain control (“AGC”) loops can also create difficulties because the receiver must immediately ‘acquire’ hop channels. Moreover, a base station receiving frequency-hopped transmissions must either implement a frequency-agile radio for each user (e.g., Bluetooth® or a GSM subscriber unit) or a bank of receivers for each hopping frequency (e.g., a GSM base station). It is appreciated by those skilled in the art that either implementation would be expensive if one wants to support multiple users. Because of the aforesaid and other reasons, European Telecommunication Standards Institute (“ETSI”) adopted a CDMA approach for European 3G cellular over the frequency hopping approach used by GSM for 2G cellular.
Therefore, there is a need for a technique that would enable IEEE 802.16 single carrier systems and other single carrier systems to benefit from the power concentration of a narrowband signal, while still retaining the full bandwidth and frequency diversity of a broadband signal. In addition, unlike CDMA, the desired solution would support multiple simultaneous but non-interfering transmissions that might use higher order quadrature amplitude modulation (“QAM”) modulations.